1. Field of the Invention
The present invention relates to a light emitting device using a light emitting element which has a layer containing an organic compound between a pair of electrodes and which can give fluorescence or luminescence by receiving an electric field. The light emitting device referred to in the present specification is an image display device, a light emitting device or a light source. Additionally, the following are included in examples of the light emitting device: a module wherein a connector, for example, a flexible printed circuit (FPC) or a tape automated bonding (TAB) tape, or a tape carrier package (TCP)) is set up onto a light emitting element; a module wherein a printed wiring board is set to the tip of a TAB tape or a TCP; and a module wherein integrated circuits (IC) are directly mounted on a light emitting element in a chip on glass (COG) manner.
2. Related Art
A light emitting element is an element which emits light by receiving an electric field. It is said that the luminescence mechanism thereof is based on the following: by applying a voltage to a layer comprising at least one organic compound sandwiched between electrodes, electrons injected from the cathode and holes injected from the anode are recombined in the layer comprising at least one organic compound to form molecules in an exciting state (hereinafter referred to as “molecular exciton”); and energy is radiated when the molecular exciton moves back toward the ground state thereof.
The kind of the molecular exciton which is made from the organic compound may be a singlet exciton state or a triplet exciton state. In the present specification, luminescence (that is, light emission) may be based on the contribution of any one of the two.
In such a light emitting element, its layer comprising at least one organic compound is usually made of a thin film having a thickness below 1 μm. The light emitting element is a spontaneous light type element, wherein the layer comprising at least one organic compound itself emits light. Therefore, backlight, which is used in conventional liquid crystal displays, is unnecessary. As a result, the light emitting element has a great advantage that it can be produced into a thin and light form.
The time from the injection of carriers to the recombination thereof in the layer comprising at least one organic compound having a thickness of about 100 to 200 nm is about several tens nanoseconds in light of carrier mobility in the layer comprising at least one organic compound. A time up to luminescence, which includes the step from the recombination of the carrier to luminescence, is a time in order of microseconds or less. Therefore, the light emitting element also has an advantage that the response thereof is very rapid.
The light emitting element draw attention as next generation flat panel display element due to the characteristics of thin and light weight, high responsibility, and direct low voltage driving. Visibility of the light emitting element is comparatively good because the light emitting element is a self-emission type and wide viewing angle. Thus, the light emitting element is considered as an effective element for using a display screen of a portable apparatus.
In light emitting devices formed by arranging such light emitting elements in a matrix form, driving methods called passive matrix driving (simple matrix type) and active matrix driving (active matrix type) can be used. However, in the case in which the density of pixels increases, it is considered that the active matrix type wherein a switch is fitted to each pixel (or each dot) is more profitable since lower voltage driving can be attained.
Moreover, as an active-matrix type light emitting device shown in FIG. 17, it has the light emitting element 1707 in which TFT 1705 on a substrate 1701 and the anode 1702 are electrically connected, a layer comprising at least one organic compound 1703 is formed on an anode 1702, and a cathode 1704 is formed on the layer comprising at least one organic compound 1703. In addition, as anode materials in the light emitting element 1707, in order to make hole injection smooth, conductive materials of a large work function is used, and conductive materials that are transparent to the light, such as ITO (indium tin oxide) and IZO (indium zinc oxide), are used as a material which fulfills the practical characteristic. The light generated at the organic light emitting layer 1703 of the light emitting element 1707 radiates toward the TFT 1705 through the anode 1702 is a favored structure (hereinafter referred to as a bottom emission) of the light emission.
However, in the bottom emission structure, even if resolution is tried to be raised, TFT and wiring may be interfered due to their arrangement. Thus, a problem of a restriction of an aperture ratio is occurred.
In recent years, the structure that the light radiates upward from the cathode side (hereinafter referred to as a top emission) is devised. Concerning to the top emission light emitting device is disclosed in unexamined patent publication No. 2001-43980. In the case of the top emission type, the aperture ratio can be enlarged than that in the bottom emission type, so that the light emitting element which can obtain higher resolution can be formed.
However, in the case of the top emission light emitting device, if anode materials that is transparent to the light is used as conventionally, light is emitted from not only the cathode side but also the anode side, thereby the light emitting efficiency is lowered.
If the film having light blocking effect against the light emitted from the anode side is formed, one more manufacturing step must be added.
Further, if the anode is formed by using metal materials having a light blocking effect, additional manufacturing steps are unnecessary. However, these materials have smaller work function and spend higher material cost than ITO which is used conventionally. In case of using metal materials for the anode, an adhesion of the anode and the organic compound film is deteriorated in comparison with ITO.